1. Field of the Invention
The present invention pertains to a high speed analog-to-digital converter.
2. Description of the Prior Art
Electronic intelligence (ELINT) systems utilize compressive receivers to detect and separate input signals from threat radars. In a particular ELINT system described in an article entitled "ELINT Design Melds Classic Methods" by Charles Konig in Microwave & RF September 1984, pp. 150, 151, 155 and 225, a voltage-controlled oscillator sweeps the band of interest and converts it to a 500 MHz wide intermediate frequency (IF). In a compressive receiver section of the ELINT system, a sweeping local oscillator (SLO) generates a linear FM chirp signal that mixes with the incoming signal. The resulting output signal passes through a surface acoustic wave (SAW) compression filter that has a negative frequency-versus-delay slope which matches the chirp of the resulting output signal. Consequently, the sweep produces a time-compressed pulse at the output of the SAW filter for each signal in its passband. During the SLO sweep, each frequency component present in the RF input signal moves across the IF bandwidth and appears at the input to the dispersive SAW filter as a linear FM signal. When the sweep rate of the SLO is equal to the delay slope of the dispersive filter, a time-compressed pulse occurs at the filter output for every frequency component of the input. The position of a compressed pulse in time, referenced to the beginning of the SLO sweep, corresponds to the carrier frequency of an incoming pulse. Thus a spectrum analysis or impulse response of the input signal is produced at the output of the SAW filter. An analysis bandwidth of 500 MHz creates compressed pulses as narrow as 2 ns in the frequency channels. A parameter measurement unit (PMU) tags each signal's time of arrival (TOA), measures its amplitude and pulse width, and determines its frequency and electrical phase angle for calculation of the angle of arrival (AOA).
Thus, the output of a compressive receiver, in the above application, is a series of analog data pulses having a 2 nanosecond (ns) pulsewidth at 4 db, which pulses occur on the order of 10 ns apart. The amplitude of the analog data pulses corresponds to such data as the phase of the angle of arrival of the incoming signals.
To suitably analyze these compressive receiver output pulses, it is necessary to convert each analog data pulse into its digital equivalent. This requires apparatus which can effectively sample such 2 ns pulses. In order to effectively sample such pulses, a sampling rate of at least 500 MHz is required. Unfortunately, 500 MHz analog-to-digital converters which provide six or eight bit quantization outputs for such pulses are either not currently available or are very expensive.